The drive for ever smaller features sizes in surfaces and coatings has been brought about through a desire to achieve attractive properties for applications in, for example micro-electronics, anti-reflective coatings for displays and super-hydrophobic or even self cleaning surfaces. The numerous techniques for preparation of nano-structured surfaces or coatings can be regarded as fitting into one of two classes.
The first class is known as the top down approach and is primarily concerned with the manufacture of progressively smaller and smaller feature sizes in photolithographic processes. Top down lithographic techniques are especially applied where regular and straight features are required as is the case for photo-resists in the microprocessor field. Manufacture of random, irregular structures and structuring via these techniques over large surface areas (dm2 to m2) is extremely difficult, time consuming and therefore expensive.
The second class of nano-structuring techniques can be regarded as so-called bottom up techniques and are usually based on phase separation, which can be achieved in two ways. Firstly, by phase separation of two incompatible polymers (e.g. Polymethylmethacrylate and Polystyrene, Steiner et al., Science, Vol 283, pg 520-522, 1999), during the evaporation of a solvent. After that the nano-structure is formed by removal of one of the polymers by a selective solvent, that dissolves only one of the polymers.
Secondly, phase separation can be induced by crosslinking a mixture of components where at least one component is capable of crosslinking and at least one other is not (Ibn-Elhaj and Schadt, Nature, Vol 410, pg 796-799, 2001). The mixture is homogeneous prior to crosslinking and the phase separation occurs during the formation of the crosslinked network. The nano-structure is formed by removal of the non-crosslinkable component. As phase separation is a dynamic process, the size and distribution of the features formed under both of the above techniques is strongly dependent on the rate or kinetics of the phase separation. Parameters such as temperature and in the case of photo-induced crosslinking the photo-initiator concentration and/or radiation intensity all will affect the feature size of the nano-structured coatings. Therefore processing is complicated and reproducibility is poor. Also the necessary washing to remove one of the components is a sizable hurdle in the commercialisation of such technologies, especially for large surface areas. Thus, there is a need for an simple technique that allows the manufacture of nano-structured coatings.
Preparation of surface nano-structured and/or nano-porous coatings or films can be achieved by a process which includes the steps of
a) applying a mixture, which mixture comprises
                i) reactive nano-particles, having reactive organic groups on their surface        ii) a solvent        iii) optionally a compound having at least one polymerisable group in an amount that is small enough to allow the formation of a nano-structured surface after cure of the formulation,to a substrate,b) polymerising the mixture on the substrate.        
By using no or only a little of the compound having at least one polymerisable group, a coating or a film is obtained with the process of the present invention having a nano-structured surface. Furthermore the coating or film has favourable mechanical properties and adheres well on several substrates. The coating or film has a variety of applications, among which are films or coatings having super hydrophobic properties and especially coatings having anti-reflective properties.
A new process for applying an antireflective coating to a substrate is also disclosed. The process comprises the steps of                a) applying a first coating to the substrate, said first coating being cross-linkable,        b) partially cross-linking the first coating,        c) providing a second coating, said second coating being cross-linkable and being carried in a solvent capable of partially swelling the first coating,        d) applying the second coating and solvent to the partially cross-linked first coating, and        e) cross-linking the second coating.The first coating can be a hard coat and the second coating a layer with a nano-structured surface as described herein. By partially cross-linking the first coating and swelling it with a solvent, good adhesion between the first and second coatings can be achieved. As a result, use of the process of the present invention can result in a durable, scratch-resistant antireflective coated substrate which is more readily manufacturable, is produced using fewer processing steps and in higher yields, and is less expensive than conventional antireflective coatings.        
From U.S. Pat. No. 2,601,123 a coating for reducing surface reflections is known for the coating of glass substrates. The coating however must be sintered at very high temperatures, so that it is not suitable for application on polymer substrates. Furthermore it relates to very old and abandoned technology.
It is known from U.S. Pat. No. 5,582,859 that anti-reflective properties may be obtained by preparing a multilayer system of coatings in which each coating has a carefully selected thickness and refractive index. However, multilayer coatings suffer from two sets of problems. The first is that the anti-reflective performance of multilayer coatings suffers from angle-dependency (D. Chen, Solar Energy Materials & Solar Cells, 86 (2001) 313-336). This means that transmission will vary from the normal to oblique angles. Secondly, reproducible processing of such multilayer coatings with precisely controlled thickness and optical properties is difficult and therefore costly and time-consuming.
Also in U.S. Pat. No. 6,455,103 a similar multi-layer anti-reflective coating is applied, giving the same problems as explained above.
In DE 19746885 A1 a process for coating a substrate with a mixture comprising reactive nanoparticles is disclosed. However there is no teaching given how to obtain the surface nano-structured and/or nanoporous coating of the present invention and the anti-reflective properties are poor.
Since the reactive nano-particles in step a) always have more than one reactive group on their surface, the polymerisation in step b results in the formation of a crosslinked phase, comprising the nanoparticles interconnected. Therefore, in this text, the terms polymerise or polymerisation is used interchangeably with cross-linking or cross-linked and polymerisable group is used interchangeably with cross-linking group.
Within the context of the invention the term surface nano-structured coatings refers to coatings having a surface roughness, and in which surface the dimensions of the features on the surface are larger than or equal to the smallest cross-sectional dimension of an individual nano-particle. Preferably the dimensions of the features are smaller than 1200 nm, more preferably smaller than 800 nm, still more preferably smaller than 400 nm, most preferably smaller than 300 nm. There also may be nanosized voids in the coating.